Lynch syndrome (LS; also referred to as hereditary non-polyposis colorectal carcinoma, HNPCC) is an autosomal dominant cancer predisposition syndrome caused by germline mutations in the DNA mismatch repair (MMR) genes (MLH1, MSH2, MSH6, and PMS2) or EPCAM, resulting in a defect in DNA mismatch repair. The LS confers 70-80% lifetime risk of developing colorectal cancer (CRC), comprising 2-5% of all CRC cases, as well as extra-colonic cancers, including cancers of endometrium, ovary, stomach, small bowel, biliary and pancreatic system, and urinary tract. It is the most common hereditary cancer syndrome. In the US alone, nearly one million individuals are estimated to be LS mutation carriers, although many are most likely unaware of their predisposition. As the uptake of genetic testing by the children of the LS mutation carriers increases, the LS is expected to become one of the largest identifiable high-risk cohorts that can benefit from effective cancer prevention interventions. In a previously completed randomized placebo-controlled trial, referred to as the Colorectal Adenoma/carcinoma Prevention Programme (CAPP)-2 trial, 600 mg aspirin per day administered for a mean of 25 months was found to substantially reduce cancer incidence in LS carriers after long-term follow-up. However, the optimal treatment regimen has yet to be fully established. It is also unclear if and what adverse events may be associated with long-term treatment. Development of safer, more efficacious and innovative preventive measures should be pursued for the prevention of LS-associated cancers. DNA MMR deficiency leads to the accumulation of insertions and deletions in short repetitive DNA sequences, called microsatellites (MS), which are particularly prone to DNA polymerase slippage during DNA replication, resulting in the phenotype called microsatellite instability (MSI). Tumors in LS patients are characterized by high levels of MSI (termed MSI-H) associated with dense immune cell infiltrates, indicating that LS MSI-H tumors are highly immunogenic. Dr. Magnus von Knebel Doeberitz and his colleagues from University of Heidelberg have previously reported that T cells isolated from MSI-H colorectal cancer (CRC) specifically recognized frameshift peptides (FSP), which resulted from shifts of the translational reading frames within the coding MS regions due to MSI-related indel mutations. They proposed that dense immune cell infiltrates in MSI-H CRC and the presence of FSP-specific T cell responses observed in the LS patients were indicative of the host immune responses to FSP-expressing CRC cells. Interestingly, T cell reactivity to FSP were already demonstrated in the peripheral blood of healthy LS mutation carriers, who had not developed CRC or other cancers, suggesting the presence of FSP-directed immune surveillance in LS carriers. To boost the host immune responses to FSP-expressing CRC and thus to enhance immune-mediated control of tumor growth, Dr. von Knebel Doeberitz and his colleagues developed FSP-based cancer vaccine, termed MicOryx, consisting of three FSP neo-peptides (AIM2(-1), HT001(-1), TAF1B(-1)) commonly detected in MSI-H CRC. Preliminary analysis of the results from the Phase I/IIa trial (NCT01461148) demonstrated that the MicOryx vaccine induced strong humoral and CD4+ T-dominant cellular immune responses, while causing no vaccine associated severe adverse events in the vaccinated patients with MSI-H CRC. They suggested that FSP antigen-based vaccination strategies would be a useful option for cancer treatment and prevention in LS mutation carriers. Dr. Steven Lipkin from Weill Medical College of Cornell University and Dr. Magnus von Knebel Doeberitz submitted a concept application to the NCI PREVENT Program, proposing to develop a cancer preventive vaccine with FSP neo-peptides used as immunogens used alone or in combination with NSAID agent(s) for LS mutation carriers. The current study aims to develop and optimize FSP vaccination strategies using a preclinical model of LS-associated gastrointestinal cancer toward clinical translation.